1. Field of the Invention
The invention pertains to the field of methods of access to a communications channel working in time-division mode, especially for a traffic management system.
2. Description of the Background
There currently exist studies of anti-collision systems or traffic management systems, relating for example to air traffic or sea traffic, in which each moving body participating in the system periodically sends out a standardized message. This message has information on the geographical position of the moving body and, if necessary, other information used for avoidance maneuvers, such as information relating to course, speed, next course, etc. A particular embodiment of a system such as this is described in the French patent application No. FR 92.03714 filed on behalf of the present Applicant. According to this mode, the time is conventionally divided into time periods T, and each period is itself divided into a number N of slots, each slot being referenced by an index 1, 2, 3 . . . i . . . N. Synchronization is maintained in a known way between the different moving bodies participating in the system, by the distribution of a universal time that is broadcast from a station whose position is known. Since each moving body knows its own position, it can get synchronized by taking account of the time taken by the synchronization information to travel between itself and the broadcasting station. The assigning of a transmission slot to a moving body participating in the system is done from a central station, for example one of the users.
This then results in an optimal use of the channel, whose load may approach 100% (giving a channel that is permanently occupied, with neither idle time, nor the collision of messages).
This approach, known as TDMA (Time Division Multiple Access) is not always desirable (because of the vulnerability of the "conductor") or possible (because of swarming by the mobile users).
This is why it is also provided, in certain systems, that access to the network will be got randomly. Each participant listens on the frequency and identifies the occupied slots, and then transmits by making a random choice of one of the unoccupied slots, hoping that no new (or former) user would have chosen this very same slot.
The success rate is high when the channel is not at all busy. The risk of collision among messages (from different users) increases with the load of the channel and tends towards the pollution of all the messages. Statistical computations indicate a value equal to 1/e (i.e. 36.8%) as being the maximum permissible load.
This technique of access is known as the slotted ALOHA technique. As compared with an access technique without slotting and without synchronization, the slotted technique enables the capacity of the channel to be doubled because there is no partial overlapping. A message is either entirely polluted or not polluted at all.
The method according to the invention can be applied to a slotted ALOHA type system. It is aimed at increasing the real capacity of the channel, all other conditions, especially the lengths of the periods and the slots, being furthermore equal. In a particular embodiment, it is also aimed at enabling each moving body participating in the system to ascertain that the slot on which it is transmitting is not polluted by another user.
The method according to the invention is of the slotted ALOHA type. It is therefore a method in which the user mobile bodies are synchronized and in which the time is divided into periods with a duration T, each period being itself divided into N slots. One user message takes up one time slot. Access to the network is obtained after at least one listening period designed to identify the unoccupied slots, by a random choice of a slot identified as being unoccupied. It is specified that, in this type of known method as in the method according to the invention, each moving body transmits at least once per period and possibly several times if the frequency needed to produce its message is higher than the repetition frequency of the period T. In a known way, the repetition frequency of the message of a moving body increases with the proximity of this moving body to another moving body. When a moving body is at a great distance from other moving bodies, it is possible to reduce the frequency of its message.
The value of the period is defined precisely as a function of the lowest need, corresponding then to only one transmission per period.
This procedure makes it possible to stabilize the frame of the period. The frame of the period is constituted by the real presence of transmission in certain slots of the period. In order to facilitate access by new users, it is worth seeing to it that this frame is stable. A frame will be said to be stable if it changes only slowly. The ideal situation would be one wherein the frame changes only at the end of the occupancy of slots by the moving bodies leaving the zone and the occupancy of unoccupied slots by the moving bodies coming into the zone or by the moving bodies that are already present in the zone but need to increase the frequency of transmission of their message. In the known methods, owing to the mode of access to the network, no moving body is sure that the slot that it has chosen has not been chosen simultaneously by another moving body.
If this is so, the moving bodies transmitting on the same slot have no possibility of realizing that their own messages have been polluted by the simultaneous presence of other messages. In order to shorten the duration of simultaneous transmissions if any, and reduce their probability of occurrence, provision is made in the known methods for the moving bodies participating in the network to themselves change their slots from time to time. This possibility of changing slots increases the probability that one and the same unoccupied slot will be chosen by several moving bodies. The same unoccupied slot can be chosen not only by moving bodies gaining access to the network or to an additional slot but also by participants who merely wish to change their slot.
The method according to the invention is aimed at providing greater stability to the transmission frame. This stability of the frame results in easier access to the network by reducing the probability that one and the same unoccupied slot will be chosen by several moving bodies. In a complementary way, the invention is aimed at limiting changes of slots by a user already occupying a slot. The changing of a slot will occur only when the same slot is really occupied by at least one other moving body that is a user of the network. Owing to the above-mentioned advantages, the method enables the real capacity of the network to be increased.
The invention can be applied not only for the communications channel of an anti-collision system but also for any communications system using a single channel in time-division mode which each user uses only in ease of need. Depending on the urgency of the transmission of its message, the user will be led to search for one or more time slots within each period.